Commonly assigned U.S. Pat. No. 5,268,708, which issued to Harshbarger et al. on Dec. 7, 1993, discloses an image processing apparatus arranged to form an intended image on a receiver secured to the periphery of an imaging drum while the drum is rotated past a printhead. A translation drive then traverses the printhead axially along the imaging drum, in coordinated motion with the rotating imaging drum. A scanning subsystem or write engine provides the scanning function by generating a once per revolution timing signal to data path electronics as a clock signal while the translation drive traverses the printhead.
The translation drive motion is obtained using a DC servo motor with a feedback encoder. The DC servo motor rotates a lead screw that is aligned generally in parallel with the axis of the vacuum imaging drum. The DC servo drive motor induces rotation to the lead screw moving the translation stage member and print head along the threaded shaft as the lead screw is rotated. The lateral directional movement of the print head is controlled by switching the direction of rotation of the DC servo drive motor and thus the lead screw.
Although the presently known and utilized image processing apparatus is satisfactory, it is not without drawbacks. The DC servo motor that is used to drive the lead screw requires feedback control signals from an expensive, high-precision encoder. Control circuitry must accept the encoder signal as input and process this feedback signal to obtain the correct output signal for driving the DC servo motor. The need for these added components increases the cost and design complexity of the image processing apparatus.
As an alternative method for providing precise rotational positioning, a stepper motor can be employed. Stepper motors provide precise rotational motion that can be used to rotate a lead screw device in order to provide precise linear motion. The stepper motor has a shaft motion characterized by the capability to achieve discrete angular movements of uniform magnitude based on its input signal. In its simplest implementation, this type of motor is driven by a sequentially switched DC power supply that provides square-wave current pulses rather than analog current values.
Internally, the stepper motor uses magnetic attraction and repulsion of a rotor in discrete steps so that the rotor takes an angular orientation at some integral multiple of a divisor angle that is based on the number and position of stator teeth and on rotor characteristics. To achieve this controlled motion, the stepper motor has two separate windings (A and B). The drive components for the stepper motor coordinate the timing of current to each set of windings so that different internal stator poles have different magnetic states for each rotor position. In a "full step current, 2-phase on" mode, windings A and B are independently energized in one of two discrete current levels, at full current. This arrangement provides highly precise positioning for most stepper motors to, typically, four hundred steps per rotation. With four hundred steps per rotation, each step moves the rotor 0.9 degrees.
For some applications, such as in an image processing apparatus, however, finer resolution than four hundred steps per revolution is required. To achieve finer resolution from the stepper motor and lead screw design, there would be significant physical requirements and cost. For example, using a finer lead screw resolution requires that the drive motor accelerate and run at faster speeds than may be practicable for rapid starting and stopping. This requirement for higher speeds also complicates synchronization between the print head traversal subsystem and the vacuum drum motor. To overcome this and other limitations, the stepper motor can be used in microstepping mode. This uses the fact that variable amounts of current through stator windings in turn vary the amount of magnetic force in the stator pole. This allows the rotor to take intermediate angular positions, between the discrete "step" positions described earlier.
In microstepping mode, the phase current exhibits a voltage-time relationship with discrete steps such that the composite waveform is sinusoidal. With microstepping, the A and B phases are substantially two sine waves with 90 degrees phase shift from each other. Since the rotor position adjusts in some proportion to the magnetic force from stator windings, this allows the rotor to take intermediate positions. This arrangement gives the stepper motor many times the positioning resolution of discrete stepping using square wave current input. Typically, the upper range achievable using microstepping is about five hundred microsteps per step. For a motor with four hundred steps per revolution, for example, this would allow two hundred thousand microsteps per revolution.
The tradeoffs with microstepping include variable torque, since different levels of current are flowing for each different position. In addition, since stator windings are energized at some intermediate current level, rather than at full current, rotor position is not as stable as with full step mode. Hence, the accuracy of each microstep is not as precise as is accuracy for full steps. Typically, feedback loops are employed to improve positioning as compensation for this loss of positional accuracy when using microstepping. However, feedback loops require costly design effort and precision feedback components.
The mechanism for print head positioning in image processing apparatus must overcome the inherent inaccuracy in microstepping, as described above. This presents particular difficulty for the process of synchronizing print head positioning at the beginning of each swath. Any additive error that accumulates over the length of the image may cause sizing problems, banding, or other objectionable image anomalies.
There has been widespread use of stepping motors and microstepping, including circuitry components specifically designed to allow modification of the current using look-up tables or other control means to effectively shape the phase current waveform (Example: Compumotor OEM650 Motor Drive User Guide gives jumper settings for modifying motor waveform shape). Existing techniques also allow modification of the third harmonic frequency of the current waveform, attenuating this harmonic component to smooth the delivered signal which alters the composite current waveform from its normal sine wave characteristic.
Reference materials showing microstepping include Compumotor Catalog, Step Motor & Servo Motor Systems and Controls, Parker Motion & Control, Rohnert Park, Calif.; Compumotor OEM650 Drive and Drive/Indexer User Guide; P/N 88-013157-02A, Compumotor Division, Parker Hannifin Corporation, Rohnert Park, Calif.; and Data Sheet, IM2000 High Performance Microstepping Controller, Intelligent Motion Systems, Inc., Taftville, Conn. Patents that disclose methods for increasing the accuracy of a stepper motor in microstepping mode include U.S. Pat. No. 4,710,691, which issued to Bergstrom et al. on Dec. 1, 1987, and discloses use of a special apparatus to characterize positional error and correct this error by a process of measurement, adjustment, re-checking, and storing the corrected phase winding current values in memory; U.S. Pat. No. 4,584,512, which issued to Pritchard on Apr. 22, 1986, and discloses the use of harmonic frequencies of the stepper motor windings current to adjust motor resonance; and U.S. Pat. No. 4,115,726, which issued to Patterson, et al. on Sep. 19, 1978, and discloses the use of odd harmonics for stepping motor compensation.